Known in the past has been an air-fuel ratio control system of an internal combustion engine, in which an exhaust purification catalyst is arranged inside an exhaust passage of the engine, an upstream side air-fuel ratio sensor is arranged upstream of the exhaust purification catalyst inside the engine exhaust passage, a downstream side oxygen sensor is arranged downstream of the exhaust purification catalyst inside the engine exhaust passage, and the amount of feed of fuel to the engine is feedback controlled, based on the output signals of the upstream side air-fuel ratio sensor and the downstream side oxygen sensor, so that the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst becomes the target air-fuel ratio, for example, the stoichiometric air-fuel ratio (for example, see PTL 1). In this air-fuel ratio control system, the basic fuel injection amount which is required for making the air-fuel ratio the stoichiometric air-fuel ratio is stored in advance. For example, this basic fuel injection amount is multiplied with a feedback correction coefficient to calculate the actual injection amount.
In this case, this feedback correction coefficient is controlled based on the output of the upstream side air-fuel ratio sensor so that the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst becomes the stoichiometric air-fuel ratio. On the other hand, sometimes, even if performing such feedback control, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst deviates from the stoichiometric air-fuel ratio. In this case, in order to correct the deviation in the air-fuel ratio from the stoichiometric air-fuel ratio, a correction value of the feedback correction coefficient which is required for making the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst the stoichiometric air-fuel ratio is obtained, as a learning value, based on an output signal of the downstream side oxygen sensor. This learning value is used to correct the feedback correction coefficient.
In the meantime, in this air-fuel ratio control system, when the downstream side oxygen sensor is normal, the air-fuel ratio detected by the downstream side oxygen sensor does not continue to deviate to the rich side. Therefore, when the air-fuel ratio detected by the downstream side oxygen sensor continues to deviate to the rich side, it is judged that the downstream side oxygen sensor is abnormal. Similarly, in this air-fuel ratio control system, when the downstream side oxygen sensor is normal, the air-fuel ratio which is detected by the downstream side oxygen sensor does not continue to deviate to the lean side. Therefore, when the air-fuel ratio detected by the downstream side oxygen sensor continues to deviate to the lean side, it is judged that the downstream side oxygen sensor is abnormal.